COCOA — What a difference a day makes in Cocoa, one of two Florida cities still operating on a four-day workweek.

The city is close to an impasse in negotiations with the union representing city workers from secretaries to public works employees, and the workweek issue is a major stumbling block, sources close to the talks said.

Because negotiations are in progress and the city council already unanimously endorsed a five-day workweek, city officials were reluctant to discuss the pros and cons of the change Monday.

But Mayor Dollye Robinson said she supports returning to a five-day week because several people told her during her campaign for office that they opposed city hall being closed Fridays.

''We've had a lot of complaints from taxpayers. It's inconvenient and I can't see how it is saving the city any money,'' she said. The four-day week began in 1979 to conserve energy, she said. City employees work 10-hour shifts.

Jerry King, city personnel director and the city's main negotiator, said union officials have requested a federal mediator to help head off an impasse. King said three or four issues in the contract, which should have been in place by Oct. 1, remain in dispute. There is disagreement over whether employees will receive wage increases retroactive to Oct. 1, he said.

About 150 employees are covered by the union contract.

Miramar, northwest of Miami, is the only other city with a population of more than 10,000 still using the four-day workweek, according to the Florida League of Cities. Pembroke Pines, a city of 55,000 south of Fort Lauderdale, was on a four-day schedule until about three years ago.

Pembroke Pines City Manager Woody Hampton said his city's experiment with the four-day week ''worked very good. It was a modern approach to management.''

He said less sick leave was used, much less overtime was recorded and greater productivity and better morale resulted.

''But we had an election and the new council ran on a platform of reinstituting a five-day workweek, so that's what we've got now -- a work schedule set by politics,'' he said.

One advantage of the four-day week is projects that require more time can be completed in one day, Hampton said. For example, if a pump at a sewer system lift station goes down, it can be repaired in nine or 10 hours, or one day on the four-day system.

''But if you are working only eight hours, then the first day you get almost finished, then you quit. That night you have to use an auxiliary pump, which

costs more. The next morning the worker has to go to the office first, get his tools, get back out to the job, etcetera, and you end up with a 10-hour job taking 14 hours,'' he said.

Hampton said it was not unusual to see employees staying late on their own initiative during the 10-hour days to make sure work was completed. Now, he said, it is like a stampede at 5:01 p.m.

City officials also found that employees scheduled personal appointments on their off days instead of taking sick leave, Hampton said.

Robinson said that may be so, but her findings indicate that taxpayers are inconvenienced by the shortened workweek.

''And what counts is what the taxpayer wants rather than what is good for the employee,'' she said.